The techniques heretofore utilized for photographing paintings have typically consisted of arrays of an equal number of tungsten lights placed at either side of the subject painting. The principal drawback of this prior art method is its inability to permit the details of a painting's texture to be reproduced. It also renders it difficult for an entire painting to be evenly lit from corner to corner without laborious setting-up. To reproduce the details of texture it is necessary for the dominant lighting source to be directed from above the painting at the preferred angle of 40.degree.-55.degree. to the perpendicular (FIG. 1) to the painting although satisfactory results may be obtained using other angles up to 80.degree.. This is not feasible with prior art methods since multiple arrays of stationary top lighting will not evenly illuminate a painting; such arrays characteristically produce a fall-off of light towards the bottom of the painting. The use of a light placed below the painting to fill in gaps in top lighting will cause the texture-highlighting benefits of top lighting to be lost.
Further problems with prior art techniques lie in obtaining faithful color reproduction while at the same time using techniques to eliminate glare spots. The film generally used for photographic reproduction of paintings has heretofore been Kodak Ektachrome tungsten film or other color films. A transparency produced using this type of film has a contrast greater than that of the subject painting. A recognized preferred medium for photographing paintings is Kodak #6121 duplicating film (presently the only gamma 1. 0 film commercially available) which will faithfully reproduce the contrast and color of a subject painting if used at an exposure of approximately 10 seconds. At greater exposures reciprocity failure occurs in this film resulting in color distortion. However, it has not hitherto been possible to utilize this material for the reason set out below.
In order to obtain enough light to photograph a painting using Kodak #6121 duplicating film (ASA of approximately 7) it would be necessary to use an array of at least six 500 watt tungsten lights. Polarizing filters must be used on each of the lights and on the camera lens in order to eliminate glare spots which will detract from the quality of the photograph, and their use necessitates an exposure increase of 21/2 f stops. Thus in order to photograph a painting by conventional techniques using an array of six 500 watt lights with polarizing filters, an exposure of 10 seconds at f8 would be required. With such an exposure, using a 16 inch lens, the length commonly employed in copying paintings, the depth of field would be approximately 3/32 inch. As in practice neither the film plane nor the subject painting is perfectly flat, it is not possible with such a narrow depth of field for the whole area of the painting to be in focus. In order to increase the depth of field to an acceptable amount with the specified lens, it is necessary to use an aperture of f22 or less. In order to do so one would have to use bulbs delivering 24 kilowatts of light. The use of such an array of lights would not be feasible in normal conditions for two major reasons. First, such an array of lights could not be used without specially adapted power circuits, and in any event it would generate an unacceptable amount of heat. Second, if it is assumed that 24 one kilowatt bulbs are used it would be necessary to align 24 polarizing filters--an impractically laborious process.
The scanning light source device which is the subject of this invention solves the foregoing problems. A light source aimed downward at an angle of between 45.degree. and 55.degree. to the horizontal and projecting onto the painting an elongated rectangle of light is moved across the subject painting in precisely planned vertical sweeps in an otherwise darkened environment. Each part of the subject painting is subjected to bright illumination for approximately 10 seconds, the optimum exposure time of Kodak #6121 duplicating film, by light reaching the painting at the desired angle for the reproduction of brush strokes and texture detail. The total time taken to photograph a painting will, of course, vary according to the size of the painting but, for example, a painting 5 feet by 7 feet may be photographed in 14 minutes. Sufficient light is cast onto the painting to enable the use of an f32 lens aperture and polarizing filters. The use of Kodak #6121 duplicating film results in the tonal range and color of the subject painting being faithfully reproduced.
The system further provides for precise timing and spacing of the required number of vertical sweeps of the light source enabling the exact amount of light calculated to be required for optimum photographic quality to be projected onto the subject painting and also making it possible for the entire surface of the planar subject to be evenly illuminated.
It is accordingly the principal object of the present invention to provide a new and improved lighting system for photographing paintings.
Another object of the present invention is to provide a lighting system which will deliver a beam of light at an angle of 40.degree.-55.degree. to the painting or other planar target enabling brush work painting techniques and surface texture to be reproduced in detail heretofore impossible.
Another object of the present invention is to provide a lighting system for photographing paintings which will enable sufficient light to be cast on the painting to be photographically reproduced to enable the entire tonal range of the painting to be recorded using gamma 1.0 film.
Still another object of the invention is to provide an illumination system which will deliver enough light to a painting so that it may be photographed using a lens aperture of f22 or smaller to maximize the ease with which a sharply focused photograph of a painting may be obtained.
Still another object of the in is to provide a lighting system making it possible to light the surface of a painting evenly from corner to corner.
A still further object of the invention is the provision of a system for illuminating paintings to be photographically reproduced which will produce greater amounts of light utilizing significantly less electricity than prior art systems.